It is reported about porous materials called monoliths in which through flow channels and a skeleton are integrated, that advantages which have not been recognized with conventional particle-filled type columns are developed, that is, when they are used as a separation medium for high-performance liquid chromatography, high column performance is obtained due to the wide flow channels and the thin skeleton, and deterioration of the performance is inhibited even under a high flow rate due to a low pressure loss (see N. Ishizuka, H. Kobayashi, H. Minakuchi, K. Nakanishi, K. Hirao, K. Hosoya, T. Ikegami and N. Tanaka, J. Chromatogr. A, 960, 85 (2002)).
Monoliths mainly comprising silica gel realize high performance due to their fine structural control, and some of them are commercially available.
While some polymer monoliths based on organic macromolecules are present, conventional polymer monoliths can not freeze the spinodal decomposition structure, which is a transitional state of a biphasic phase separation process, due to the heterogeneity of ordinary radical polymerization as mentioned later. Therefore, they are usually of particle aggregation type having more developed phase separation. Therefore, although a through pore is formed, the structure is of low uniformity and becomes particle aggregation type. As a result, high performance like that of monoliths based on silica gel has not been realized yet.
Conventionally, polymer monoliths are produced basically by combining hydrophobic monomer components such as styrene/divinylbenzene with, for example, a porogen which serves as a poor solvent for such monomers as represented by a patent to Svec, et al. (U.S. Pat. No. 5,453,185). In such a solution system, the van der Weals force between polymer chains is basically greater than the steric hindrance of growing polymer chains, resulting in flocculation of polymer chains. Thus, this causes generation of nuclei due to polymer chain entanglement, growth of microgel particles due to polymer chain flocculation, and rapid increase in surface energy of the system. Moreover, the microgel particles aggregate together, so that the gel continues similar growth (fractal) to get coarser. Therefore, such a system composed of a monomer and a poor solvent is characterized in that gel grows in the form of particle aggregation, and phase separation occurs extremely early in competition with gelation and a monolith form where macroporous pores have a small specific surface area is fixed, resulting in a particle aggregation type monolith. Therefore, conventional monoliths have no skeletal structure and inherently have some problems, such as large maze factors of a through pore, increase in back pressure at high flow rate, and morphological change due to monolith compressibility.